Outgassing test sample chamber (micro-CVCM): the vacuum chamber, heated compartment bar and cooled collectors used for ASTM E595

01

What the chamber has to hold: 125 °C sample, 25 °C collector, below 7 × 10⁻³ Pa for 24 hours

ASTM E595 describes its apparatus and procedure for subjecting materials to 125 °C at less than 7 × 10⁻³ Pa (5 × 10⁻⁵ torr) for 24 h, measuring total mass loss (TML) and the collected volatile condensable materials (CVCM) that condense on a collector at 25 °C; the water vapour regained (WVR) can be measured afterwards as an option. Tolerances are ±1 °C on 25 °C and 125 °C and ±2 °C on 23 °C. [1]

The standard explains why the vacuum need not approach interplanetary levels such as 10⁻¹² Pa: it is enough that the pressure be low enough for the mean free path of gas molecules to be long compared with the chamber dimensions. [1] Weighings are made at 23 °C and 50% relative humidity, because TML and WVR are affected by the material's capacity to gain or lose water vapour. [1]

Editorial calculation: 7 × 10⁻³ Pa is about 5 × 10⁻⁵ torr (1 torr ≈ 133.3 Pa), as the standard states. A Goddard paper describes the E595 assembly at about 10⁻⁶ torr, [4] that is about 1.3 × 10⁻⁴ Pa, some fifty times below the E595 limit.

02

Heated compartment bar, 6.3 mm exit port and a collector 12.7 mm away

NASA Goddard's guide to its outgassing database describes the apparatus in use. Each sample of 100 to 300 mg, nominally 250 mg, goes in a pre-weighed, cleaned aluminium foil boat into its own compartment of a solid copper bar; each compartment is closed by a solid copper cover so that volatiles escape only through a 6.3 mm (0.25 in) exit port. The bar, with 12 compartments, is heated to 125 °C for 24 h, and a chromium-plated collector in direct line of sight of the port, 12.7 mm (0.5 in) away, is held at 25 °C, with barriers near the collectors to prevent cross-contamination between adjacent samples. [2]

Goddard's 1969 report on the first system shows how the chamber was built: two resistance-heated, vertically mounted copper bars, each 25.5 in long with a 1 in square cross-section and 12 horizontal sample chambers, whose open rears face through a perforated barrier plate onto removable chrome-plated collector plates held at ambient temperature by a heat exchanger running silicone fluid. The bars stood inside a 10-in diameter Pyrex bell jar on a feed-through collar; 18 compartments were used for tests in a 24-hour run and four as controls. [3] Its procedure brought the bar to 125 °C in 25 minutes, held 125 ± 2 °C, and raised the bell 1.5 to 2 hours after the start of the cooling cycle. [3]

The guide notes that E595 fixes a set of critical dimensions so that similar systems give similar results. [2] ASTM adds that CVCM values are comparable only for similar collector geometry and surfaces at 25 °C. [1] Editorial note: for a purchased chamber, ask for a drawing of the compartment, port and collector geometry against the dimensions in the current E595 edition, not only the temperatures.

Editorial note: specify the number of sample compartments, the collector cooling method and its stability at 25 ± 1 °C, the bar heater control at 125 ± 1 °C, the pumping system and base pressure, and how the separator plate keeps each collector to its own sample.

03

Reading the result: TML 1.0% and CVCM 0.10% as screening levels

ASTM E595 leaves acceptance criteria to the user, noting that TML of 1.00% and CVCM of 0.10% have historically been used as screening levels for rejecting spacecraft materials. [1] Goddard's database lists, in a separate section, only the materials meeting a maximum TML of 1.0% and a maximum CVCM of 0.10%. [2]

Goddard's Scialdone and co-authors give the reasoning: some materials show mechanical and physical degradation after losing 3 to 5% of their mass, a 1% TML is detrimental to the mechanical properties of elastomers, and 0.1% condensate from 1 kg of material would cover 100 m² of surface with a uniform layer of 10⁻⁶ g/cm², about 20 monolayers. [4]

Editorial calculation: on the nominal 250 mg sample, 1.0% TML is 2.5 mg of mass loss and 0.10% CVCM is 0.25 mg collected on the plate, so the weighing system has to resolve hundredths of a milligram to classify a material near the limit. Goddard's 1969 procedure weighed sample boats on a micro-balance in a conditioned 40–50% RH environment for this reason. [3]

04

Why the E595 chamber reads lower than open-sample instruments

The exit port that makes the chamber repeatable also restricts flow. Scialdone and co-authors at Goddard tested a graphite–epoxy composite at 125 °C for 24 h in four instruments and found that the ASTM E595 apparatus gave consistently lower readings than the others; they attribute it to the flow conductance of the vent passage and the sample geometry. [4]

ASTM itself frames the test as a comparison, not a prediction: the 24-hour time does not represent years of outgassing in service, the test temperature should be raised if service temperatures exceed 65 to 70 °C — at least 30 °C above the maximum service temperature is suggested — and results at other sample or collector temperatures (tests have been run from 50 to 400 °C and with collectors at 1 to 30 °C) may be compared only with materials tested under the same conditions. Materials that condense only below 25 °C are not detected. [1]

05

What to give a supplier of an outgassing sample chamber

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Standard: [ASTM E595 edition; any other method to be run]
Capacity: [number of sample compartments; samples per run; control compartments]
Temperatures: [bar 125 °C ± 1 °C or other; collector 25 °C ± 1 °C or other range]
Vacuum: [base pressure required below 7 × 10⁻³ Pa; pump type; pump-down time]
Geometry: [compartment, exit port and collector dimensions against E595]
Weighing: [balance resolution; 23 °C / 50% RH conditioning]
Documents: [dimension drawing; temperature mapping of the bar; reference-material results]

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